The present invention relates to a vehicle with rear wheel steering control and active brake control. More particularly, the present invention relates to a vehicle wherein the rear wheel steering control and active brake control are integrated to produce optimal vehicle performance.
Recently, vehicles have been produced with brake systems that actively apply brakes to individual wheels in an effort to improve vehicle stability or handling. Many new vehicles also include rear wheel steering which enhances the vehicle maneuverability at low speed and the vehicle stability at high speed via control of the rear wheel steering angle.
However, when these two systems are included on a vehicle, the systems need to be integrated such that the control actions of both systems are not conflicting such that undesirable vehicle handling is achieved.
For example, an active brake control system compares the vehicle yaw rate measurement with a computed desired vehicle yaw rate during a vehicle maneuver, and forms an error term to decide and command a proper amount of differential braking for stabilization of the vehicle during the maneuver. If the state of rear wheel steering is not accounted for when computing the desired vehicle yaw rate, the computation may result in an erroneous value such that when compared with the vehicle yaw rate measurement there will be a significant error term even though the vehicle is handling correctly. The active brake control system in this circumstance would command an undesired differential braking which would result in poor vehicle performance.
As another example, if the rear wheel steering commands an out of phase steering with the front steering in a low speed region, or an in phase steering in a high speed region, vehicle handling and stability may be compromised. Although an out of phase steering enhances the vehicle""s maneuverability under normal driving conditions, there is a tendency to oversteer when maneuvering at a limit condition. Similarly, an in phase steering stabilizes vehicle maneuverability under normal driving conditions, but it has a tendency to promote understeer when maneuvering at a limit condition. Therefore, when a vehicle is operated at the limit conditions where active brake control usually takes effect, the active brake control and rear wheel steering systems need to be integrated such that they do not provide conflicting controls that would result in less than optimal vehicle performance.
Therefore, it is a purpose of this invention to provide an integration of rear wheel steering with a vehicle stability enhancement system such that there is a proper interchange of information between the two systems that results in an optimum vehicle performance.
The present invention is directed to an improved method for integrating a vehicle stability enhancement system and rear wheel steering. The method includes the steps of: inputting a vehicle speed, inputting a measured vehicle yaw rate, determining a front wheel steer angle, determining a rear wheel steer angle, calculating a desired yaw rate, comparing the measured yaw rate with the desired yaw rate to determine a yaw rate error term, and thereafter applying a braking force to a wheel of the vehicle to impart a yaw moment based upon the magnitude of the error term calculated and wherein the rear wheel steer angle is accounted for in calculating the desired yaw rate.
The method for integrating a vehicle stability enhancement system and rear wheel steering has the advantage of including the rear wheel steer angle in calculating a desired yaw rate, such that optimal vehicle performance can be maintained.
The method for integrating a vehicle stability enhancement system and rear wheel steering has the further advantage of providing an improved hand wheel position centering algorithm that includes the effects of rear wheel steering.
As a further advantage, the method for integrating a vehicle stability enhancement system and rear wheel steering provides rear wheel steering control during active brake control that includes terms within the algorithm to compensate for over steering and under steering.
FIG. 1 is a diagram of a vehicle having a vehicle stability enhancement system and a rear wheel steering system that are integrated in accordance with present invention;
FIG. 2 is a flow diagram for improving rear wheel steering control that includes determining of an,understeer or an oversteer condition;
FIG. 3 is a flow diagram detailing the overall control of the vehicle stability enhancement system;
FIG. 4 is a flow diagram detailing the hand wheel sensor centering algorithm;
FIG. 5 is a flow diagram detailing the differential braking control of the vehicle stability enhancement system;
FIG. 6 is a flow diagram detailing the algorithm for determining a desired yaw rate taking into account rear wheel steering variables.
FIG. 7 is a graph detailing a rear wheel gain curve plotted at various speeds.